1. Field of the Invention
The present invention relates to a chromatic aberration corrector and electron microscope.
2. Description of Related Art
In an electron microscope such as a transmission electron microscope (TEM) or a scanning electron microscope (SEM), chromatic aberration is one factor deteriorating resolution. If chromatic aberration is corrected, resolution is improved.
For example, JP-A-2010-114068 discloses a chromatic aberration corrector for correcting chromatic aberration using two multipole elements each of which has a thickness along the optical axis of an electron beam and produces an electromagnetic field. In the chromatic aberration corrector of JP-A-2010-114068, negative chromatic aberration is generated to cancel out positive chromatic aberration in an objective lens. Thus, the chromatic aberration in the objective lens is corrected.
More specifically, the chromatic aberration corrector of JP-A-2010-114068 has a first multipole element having a first thickness along the optical axis of a charged particle beam and producing a first static electromagnetic field of two-fold symmetry, a second multipole element having a second thickness along the optical axis and producing a second static electromagnetic field of two-fold symmetry, and a transfer lens disposed between the first and second multipole elements. In the first and second static electromagnetic fields, two-fold magnetic astigmatisms of the charged particle beam cancel out each other under given conditions. In the first and second multipole elements, concave lens effects are produced by a combination of aberrations in quadrupole fields having thicknesses.
In the chromatic aberration corrector of JP-A-2010-114068, an electric field is employed in addition to a magnetic field in each multipole element. In such a multipole element, field charging and quite slight voltage instability, for example, are major causes leading to a deterioration of the ultimate resolution of the electron microscope. As the length (thickness) of a multipole element taken along the optical axis is increased, the electric field undergoes these noise components over a longer distance. This increases the amount of perturbations of the electron beam. Therefore, reducing the length of the multipole element leads to an improvement of the resolution. In order to improve the resolution by reducing the length of each multipole element, it is necessary to efficiently produce negative chromatic aberration.